A liquid crystal display device is applied to various fields taking advantage of the features such as a light weight, a low-profile body, and low power consumption. In recent years, an OCB mode liquid crystal display device has been in the limelight as a liquid crystal display device which allows a viewing angle and a response speed to be improved. An OCB mode liquid crystal display device like this has a liquid crystal layer having liquid crystal molecules held between a pair of substrates; the molecules can be aligned in a bend. OCB mode like this is further improved in response speed by one digit in comparison to TN (Twisted Nematic) mode. Further, OCB mode has the advantage that it can offer a wide viewing angle because the influence of birefringence of light traveling through the liquid crystal layer can be optically compensated in accordance with the alignment of liquid crystal molecules (see e.g. Japanese Application Kokai No. 2002-202491).
As for OCB mode liquid crystal display device, it becomes possible to display in black at only a certain voltage because display is performed by means of birefringence of light. As shown in FIG. 5, in a range of Second Voltage above the optimal black display voltage, the transmittance is increased and thus, the reversal of gradation occurs. Therefore, with OCB mode liquid crystal display device, the black display voltage V is set to a value Vs of the optimal black display voltage which is a bottom of brightness; in regard to a display voltage for another color gradation, a voltage lower than the optimal black display voltage Vs (in a range of First Voltage in FIG. 5) is applied for display.
Meanwhile, as shown by a solid line in FIG. 6, the optimal black display voltage Vs(T) has a temperature characteristic that it lowers with a rise in the panel temperature T. On this account, a conventional OCB mode liquid crystal display device is equipped with a temperature sensor, and corrects the black display voltage V(T) according to a temperature Tr sensed by the sensor and therefore performs temperature compensation, as shown by a dotted line in FIG. 6 (see e.g. Japanese Application Kokai No. 2004-185027).
In such OCB mode liquid crystal display device which performs the temperature compensation, a temperature sensor is provided on a printed wiring board mounted with a drive circuit for a liquid crystal display device. Therefore, such OCB mode liquid crystal display device tends to sense a temperature higher than an actual panel temperature T owing to heat from a backlight and heat from an electronic part. On this account, a measure to correct the temperature difference ΔT and then apply a black display voltage V(Tr) has been taken conventionally. The black display voltage V(Tr) indicated by the dotted line in FIG. 6 shows the case where the temperature difference ΔT between the panel temperature T and sensed temperature Tr is 10° C. In this case, as the black display voltage V(T) is to be applied over all temperature zones at and below the optimal black display voltage Vs(T), the reversal of gradation never occurs.
However, as for an OCB mode liquid crystal display device as described above, immediately after its power source is turned on, there is neither heat from a backlight nor heat from an electronic part. Therefore, the temperature difference ΔT between a sensed temperature Tr and panel temperature T does not arise. Thus, the temperature characteristic exhibits a situation as shown by a single dot & dash line in FIG. 6 apparently.
Specifically, as a result of no temperature difference ΔT, the black display voltage is increased by an amount corresponding to an estimated temperature difference, and then it exceeds the optimal black display voltage Vs(T). Consequently, reversal of the gradation is occurred at the starting time when the power source is turned on. This is a problem that an OCB mode liquid crystal display device as described above has. For example, the black display voltage is at or above the optimal black display voltage Vs(T) and the reversal of gradation occurs around a room temperature (20 to 30° C.), as shown in FIG. 6.
Therefore, in consideration of the above-described problem, the invention aims at providing an OCB mode liquid crystal display device in which no reversal of gradation is caused not only when it is in its stable state but also at the time of activation thereof.